/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package org.skfiy.util.v8;

import java.io.Serializable;
import java.util.Collection;
import java.util.List;
import java.util.RandomAccess;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.lang.reflect.Constructor;

/**
 * Abstract base class for tasks that run within a {@link ForkJoinPool}. A
 * {@code ForkJoinTask} is a thread-like entity that is much lighter weight than
 * a normal thread. Huge numbers of tasks and subtasks may be hosted by a small
 * number of actual threads in a ForkJoinPool, at the price of some usage
 * limitations.
 *
 * <p>A "main" {@code ForkJoinTask} begins execution when it is explicitly
 * submitted to a {@link ForkJoinPool}, or, if not already engaged in a ForkJoin
 * computation, commenced in the {@link
 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or related
 * methods. Once started, it will usually in turn start other subtasks. As
 * indicated by the name of this class, many programs using {@code ForkJoinTask}
 * employ only methods {@link #fork} and {@link #join}, or derivatives such as {@link
 * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also provides a
 * number of other methods that can come into play in advanced usages, as well
 * as extension mechanics that allow support of new forms of fork/join
 * processing.
 *
 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. The
 * efficiency of {@code ForkJoinTask}s stems from a set of restrictions (that
 * are only partially statically enforceable) reflecting their main use as
 * computational tasks calculating pure functions or operating on purely
 * isolated objects. The primary coordination mechanisms are {@link #fork}, that
 * arranges asynchronous execution, and {@link #join}, that doesn't proceed
 * until the task's result has been computed. Computations should ideally avoid
 * {@code synchronized} methods or blocks, and should minimize other blocking
 * synchronization apart from joining other tasks or using synchronizers such as
 * Phasers that are advertised to cooperate with fork/join scheduling.
 * Subdividable tasks should also not perform blocking I/O, and should ideally
 * access variables that are completely independent of those accessed by other
 * running tasks. These guidelines are loosely enforced by not permitting
 * checked exceptions such as {@code IOExceptions} to be thrown. However,
 * computations may still encounter unchecked exceptions, that are rethrown to
 * callers attempting to join them. These exceptions may additionally include {@link
 * RejectedExecutionException} stemming from internal resource exhaustion, such
 * as failure to allocate internal task queues. Rethrown exceptions behave in
 * the same way as regular exceptions, but, when possible, contain stack traces
 * (as displayed for example using {@code ex.printStackTrace()}) of both the
 * thread that initiated the computation as well as the thread actually
 * encountering the exception; minimally only the latter.
 *
 * <p>It is possible to define and use ForkJoinTasks that may block, but doing
 * do requires three further considerations: (1) Completion of few if any
 * <em>other</em> tasks should be dependent on a task that blocks on external
 * synchronization or I/O. Event-style async tasks that are never joined (for
 * example, those subclassing {@link
 * CountedCompleter}) often fall into this category. (2) To minimize resource
 * impact, tasks should be small; ideally performing only the (possibly)
 * blocking action. (3) Unless the {@link
 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly blocked
 * tasks is known to be less than the pool's {@link
 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that enough
 * threads will be available to ensure progress or good performance.
 *
 * <p>The primary method for awaiting completion and extracting results of a
 * task is {@link #join}, but there are several variants: The {@link Future#get}
 * methods support interruptible and/or timed waits for completion and report
 * results using {@code Future} conventions. Method {@link #invoke} is
 * semantically equivalent to {@code fork(); join()} but always attempts to
 * begin execution in the current thread. The "<em>quiet</em>" forms of these
 * methods do not extract results or report exceptions. These may be useful when
 * a set of tasks are being executed, and you need to delay processing of
 * results or exceptions until all complete. Method {@code invokeAll} (available
 * in multiple versions) performs the most common form of parallel invocation:
 * forking a set of tasks and joining them all.
 *
 * <p>In the most typical usages, a fork-join pair act like a call (fork) and
 * return (join) from a parallel recursive function. As is the case with other
 * forms of recursive calls, returns (joins) should be performed
 * innermost-first. For example, {@code a.fork();
 * b.fork(); b.join(); a.join();} is likely to be substantially more efficient
 * than joining {@code a} before {@code b}.
 *
 * <p>The execution status of tasks may be queried at several levels of detail:
 * {@link #isDone} is true if a task completed in any way (including the case
 * where a task was cancelled without executing); {@link #isCompletedNormally}
 * is true if a task completed without cancellation or encountering an
 * exception; {@link #isCancelled} is true if the task was cancelled (in which
 * case {@link #getException} returns a
 * {@link java.util.concurrent.CancellationException}); and
 * {@link #isCompletedAbnormally} is true if a task was either cancelled or
 * encountered an exception, in which case {@link
 * #getException} will return either the encountered exception or
 * {@link java.util.concurrent.CancellationException}.
 *
 * <p>The ForkJoinTask class is not usually directly subclassed. Instead, you
 * subclass one of the abstract classes that support a particular style of
 * fork/join processing, typically {@link
 * RecursiveAction} for most computations that do not return results,
 * {@link RecursiveTask} for those that do, and {@link
 * CountedCompleter} for those in which completed actions trigger other actions.
 * Normally, a concrete ForkJoinTask subclass declares fields comprising its
 * parameters, established in a constructor, and then defines a {@code compute}
 * method that somehow uses the control methods supplied by this base class.
 *
 * <p>Method {@link #join} and its variants are appropriate for use only when
 * completion dependencies are acyclic; that is, the parallel computation can be
 * described as a directed acyclic graph (DAG). Otherwise, executions may
 * encounter a form of deadlock as tasks cyclically wait for each other.
 * However, this framework supports other methods and techniques (for example
 * the use of {@link java.util.concurrent.Phaser}, {@link #helpQuiesce}, and
 * {@link #complete}) that may be of use in constructing custom subclasses for
 * problems that are not statically structured as DAGs. To support such usages a
 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short} value
 * using {@link #setForkJoinTaskTag} or {@link
 * #compareAndSetForkJoinTaskTag} and checked using {@link
 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use these
 * {@code protected} methods or tags for any purpose, but they may be of use in
 * the construction of specialized subclasses. For example, parallel graph
 * traversals can use the supplied methods to avoid revisiting nodes/tasks that
 * have already been processed. (Method names for tagging are bulky in part to
 * encourage definition of methods that reflect their usage patterns.)
 *
 * <p>Most base support methods are {@code final}, to prevent overriding of
 * implementations that are intrinsically tied to the underlying lightweight
 * task scheduling framework. Developers creating new basic styles of fork/join
 * processing should minimally implement {@code protected} methods {@link #exec}, {@link
 * #setRawResult}, and {@link #getRawResult}, while also introducing an abstract
 * computational method that can be implemented in its subclasses, possibly
 * relying on other {@code protected} methods provided by this class.
 *
 * <p>ForkJoinTasks should perform relatively small amounts of computation.
 * Large tasks should be split into smaller subtasks, usually via recursive
 * decomposition. As a very rough rule of thumb, a task should perform more than
 * 100 and less than 10000 basic computational steps, and should avoid
 * indefinite looping. If tasks are too big, then parallelism cannot improve
 * throughput. If too small, then memory and internal task maintenance overhead
 * may overwhelm processing.
 *
 * <p>This class provides {@code adapt} methods for {@link Runnable} and
 * {@link Callable}, that may be of use when mixing execution of
 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are of this
 * form, consider using a pool constructed in <em>asyncMode</em>.
 *
 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be used in
 * extensions such as remote execution frameworks. It is sensible to serialize
 * tasks only before or after, but not during, execution. Serialization is not
 * relied on during execution itself.
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

    /*
     * See the internal documentation of class ForkJoinPool for a
     * general implementation overview.  ForkJoinTasks are mainly
     * responsible for maintaining their "status" field amidst relays
     * to methods in ForkJoinWorkerThread and ForkJoinPool.
     *
     * The methods of this class are more-or-less layered into
     * (1) basic status maintenance
     * (2) execution and awaiting completion
     * (3) user-level methods that additionally report results.
     * This is sometimes hard to see because this file orders exported
     * methods in a way that flows well in javadocs.
     */

    /*
     * The status field holds run control status bits packed into a
     * single int to minimize footprint and to ensure atomicity (via
     * CAS).  Status is initially zero, and takes on nonnegative
     * values until completed, upon which status (anded with
     * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
     * undergoing blocking waits by other threads have the SIGNAL bit
     * set.  Completion of a stolen task with SIGNAL set awakens any
     * waiters via notifyAll. Even though suboptimal for some
     * purposes, we use basic builtin wait/notify to take advantage of
     * "monitor inflation" in JVMs that we would otherwise need to
     * emulate to avoid adding further per-task bookkeeping overhead.
     * We want these monitors to be "fat", i.e., not use biasing or
     * thin-lock techniques, so use some odd coding idioms that tend
     * to avoid them, mainly by arranging that every synchronized
     * block performs a wait, notifyAll or both.
     *
     * These control bits occupy only (some of) the upper half (16
     * bits) of status field. The lower bits are used for user-defined
     * tags.
     */
    /**
     * The run status of this task
     */
    volatile int status; // accessed directly by pool and workers
    static final int DONE_MASK = 0xf0000000;  // mask out non-completion bits
    static final int NORMAL = 0xf0000000;  // must be negative
    static final int CANCELLED = 0xc0000000;  // must be < NORMAL
    static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
    static final int SIGNAL = 0x00010000;  // must be >= 1 << 16
    static final int SMASK = 0x0000ffff;  // short bits for tags

    /**
     * Marks completion and wakes up threads waiting to join this task.
     *
     * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
     * @return completion status on exit
     */
    private int setCompletion(int completion) {
        for (int s;;) {
            if ((s = status) < 0) {
                return s;
            }
            if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
                if ((s >>> 16) != 0) {
                    synchronized (this) {
                        notifyAll();
                    }
                }
                return completion;
            }
        }
    }

    /**
     * Primary execution method for stolen tasks. Unless done, calls exec and
     * records status if completed, but doesn't wait for completion otherwise.
     *
     * @return status on exit from this method
     */
    final int doExec() {
        int s;
        boolean completed;
        if ((s = status) >= 0) {
            try {
                completed = exec();
            } catch (Throwable rex) {
                return setExceptionalCompletion(rex);
            }
            if (completed) {
                s = setCompletion(NORMAL);
            }
        }
        return s;
    }

    /**
     * Tries to set SIGNAL status unless already completed. Used by
     * ForkJoinPool. Other variants are directly incorporated into
     * externalAwaitDone etc.
     *
     * @return true if successful
     */
    final boolean trySetSignal() {
        int s = status;
        return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
    }

    /**
     * Blocks a non-worker-thread until completion.
     *
     * @return status upon completion
     */
    private int externalAwaitDone() {
        int s;
        ForkJoinPool.externalHelpJoin(this);
        boolean interrupted = false;
        while ((s = status) >= 0) {
            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                synchronized (this) {
                    if (status >= 0) {
                        try {
                            wait();
                        } catch (InterruptedException ie) {
                            interrupted = true;
                        }
                    } else {
                        notifyAll();
                    }
                }
            }
        }
        if (interrupted) {
            Thread.currentThread().interrupt();
        }
        return s;
    }

    /**
     * Blocks a non-worker-thread until completion or interruption.
     */
    private int externalInterruptibleAwaitDone() throws InterruptedException {
        int s;
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        ForkJoinPool.externalHelpJoin(this);
        while ((s = status) >= 0) {
            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                synchronized (this) {
                    if (status >= 0) {
                        wait();
                    } else {
                        notifyAll();
                    }
                }
            }
        }
        return s;
    }

    /**
     * Implementation for join, get, quietlyJoin. Directly handles only cases of
     * already-completed, external wait, and unfork+exec. Others are relayed to
     * ForkJoinPool.awaitJoin.
     *
     * @return status upon completion
     */
    private int doJoin() {
        int s;
        Thread t;
        ForkJoinWorkerThread wt;
        ForkJoinPool.WorkQueue w;
        return (s = status) < 0 ? s
                : ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                ? (w = (wt = (ForkJoinWorkerThread) t).workQueue).
                tryUnpush(this) && (s = doExec()) < 0 ? s
                : wt.pool.awaitJoin(w, this)
                : externalAwaitDone();
    }

    /**
     * Implementation for invoke, quietlyInvoke.
     *
     * @return status upon completion
     */
    private int doInvoke() {
        int s;
        Thread t;
        ForkJoinWorkerThread wt;
        return (s = doExec()) < 0 ? s
                : ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                ? (wt = (ForkJoinWorkerThread) t).pool.awaitJoin(wt.workQueue, this)
                : externalAwaitDone();
    }
    // Exception table support
    /**
     * Table of exceptions thrown by tasks, to enable reporting by callers.
     * Because exceptions are rare, we don't directly keep them with task
     * objects, but instead use a weak ref table. Note that cancellation
     * exceptions don't appear in the table, but are instead recorded as status
     * values.
     *
     * Note: These statics are initialized below in static block.
     */
    private static final ExceptionNode[] exceptionTable;
    private static final ReentrantLock exceptionTableLock;
    private static final ReferenceQueue<Object> exceptionTableRefQueue;
    /**
     * Fixed capacity for exceptionTable.
     */
    private static final int EXCEPTION_MAP_CAPACITY = 32;

    /**
     * Key-value nodes for exception table. The chained hash table uses identity
     * comparisons, full locking, and weak references for keys. The table has a
     * fixed capacity because it only maintains task exceptions long enough for
     * joiners to access them, so should never become very large for sustained
     * periods. However, since we do not know when the last joiner completes, we
     * must use weak references and expunge them. We do so on each operation
     * (hence full locking). Also, some thread in any ForkJoinPool will call
     * helpExpungeStaleExceptions when its pool becomes isQuiescent.
     */
    static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {

        final Throwable ex;
        ExceptionNode next;
        final long thrower;  // use id not ref to avoid weak cycles

        ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
            super(task, exceptionTableRefQueue);
            this.ex = ex;
            this.next = next;
            this.thrower = Thread.currentThread().getId();
        }
    }

    /**
     * Records exception and sets status.
     *
     * @return status on exit
     */
    final int recordExceptionalCompletion(Throwable ex) {
        int s;
        if ((s = status) >= 0) {
            int h = System.identityHashCode(this);
            final ReentrantLock lock = exceptionTableLock;
            lock.lock();
            try {
                expungeStaleExceptions();
                ExceptionNode[] t = exceptionTable;
                int i = h & (t.length - 1);
                for (ExceptionNode e = t[i];; e = e.next) {
                    if (e == null) {
                        t[i] = new ExceptionNode(this, ex, t[i]);
                        break;
                    }
                    if (e.get() == this) // already present
                    {
                        break;
                    }
                }
            } finally {
                lock.unlock();
            }
            s = setCompletion(EXCEPTIONAL);
        }
        return s;
    }

    /**
     * Records exception and possibly propagates.
     *
     * @return status on exit
     */
    private int setExceptionalCompletion(Throwable ex) {
        int s = recordExceptionalCompletion(ex);
        if ((s & DONE_MASK) == EXCEPTIONAL) {
            internalPropagateException(ex);
        }
        return s;
    }

    /**
     * Hook for exception propagation support for tasks with completers.
     */
    void internalPropagateException(Throwable ex) {
    }

    /**
     * Cancels, ignoring any exceptions thrown by cancel. Used during worker and
     * pool shutdown. Cancel is spec'ed not to throw any exceptions, but if it
     * does anyway, we have no recourse during shutdown, so guard against this
     * case.
     */
    static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
        if (t != null && t.status >= 0) {
            try {
                t.cancel(false);
            } catch (Throwable ignore) {
            }
        }
    }

    /**
     * Removes exception node and clears status.
     */
    private void clearExceptionalCompletion() {
        int h = System.identityHashCode(this);
        final ReentrantLock lock = exceptionTableLock;
        lock.lock();
        try {
            ExceptionNode[] t = exceptionTable;
            int i = h & (t.length - 1);
            ExceptionNode e = t[i];
            ExceptionNode pred = null;
            while (e != null) {
                ExceptionNode next = e.next;
                if (e.get() == this) {
                    if (pred == null) {
                        t[i] = next;
                    } else {
                        pred.next = next;
                    }
                    break;
                }
                pred = e;
                e = next;
            }
            expungeStaleExceptions();
            status = 0;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns a rethrowable exception for the given task, if available. To
     * provide accurate stack traces, if the exception was not thrown by the
     * current thread, we try to create a new exception of the same type as the
     * one thrown, but with the recorded exception as its cause. If there is no
     * such constructor, we instead try to use a no-arg constructor, followed by
     * initCause, to the same effect. If none of these apply, or any fail due to
     * other exceptions, we return the recorded exception, which is still
     * correct, although it may contain a misleading stack trace.
     *
     * @return the exception, or null if none
     */
    private Throwable getThrowableException() {
        if ((status & DONE_MASK) != EXCEPTIONAL) {
            return null;
        }
        int h = System.identityHashCode(this);
        ExceptionNode e;
        final ReentrantLock lock = exceptionTableLock;
        lock.lock();
        try {
            expungeStaleExceptions();
            ExceptionNode[] t = exceptionTable;
            e = t[h & (t.length - 1)];
            while (e != null && e.get() != this) {
                e = e.next;
            }
        } finally {
            lock.unlock();
        }
        Throwable ex;
        if (e == null || (ex = e.ex) == null) {
            return null;
        }
        if (false && e.thrower != Thread.currentThread().getId()) {
            Class<? extends Throwable> ec = ex.getClass();
            try {
                Constructor<?> noArgCtor = null;
                Constructor<?>[] cs = ec.getConstructors();// public ctors only
                for (int i = 0; i < cs.length; ++i) {
                    Constructor<?> c = cs[i];
                    Class<?>[] ps = c.getParameterTypes();
                    if (ps.length == 0) {
                        noArgCtor = c;
                    } else if (ps.length == 1 && ps[0] == Throwable.class) {
                        return (Throwable) (c.newInstance(ex));
                    }
                }
                if (noArgCtor != null) {
                    Throwable wx = (Throwable) (noArgCtor.newInstance());
                    wx.initCause(ex);
                    return wx;
                }
            } catch (Exception ignore) {
            }
        }
        return ex;
    }

    /**
     * Poll stale refs and remove them. Call only while holding lock.
     */
    private static void expungeStaleExceptions() {
        for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
            if (x instanceof ExceptionNode) {
                ForkJoinTask<?> key = ((ExceptionNode) x).get();
                ExceptionNode[] t = exceptionTable;
                int i = System.identityHashCode(key) & (t.length - 1);
                ExceptionNode e = t[i];
                ExceptionNode pred = null;
                while (e != null) {
                    ExceptionNode next = e.next;
                    if (e == x) {
                        if (pred == null) {
                            t[i] = next;
                        } else {
                            pred.next = next;
                        }
                        break;
                    }
                    pred = e;
                    e = next;
                }
            }
        }
    }

    /**
     * If lock is available, poll stale refs and remove them. Called from
     * ForkJoinPool when pools become quiescent.
     */
    static final void helpExpungeStaleExceptions() {
        final ReentrantLock lock = exceptionTableLock;
        if (lock.tryLock()) {
            try {
                expungeStaleExceptions();
            } finally {
                lock.unlock();
            }
        }
    }

    /**
     * A version of "sneaky throw" to relay exceptions
     */
    static void rethrow(final Throwable ex) {
        if (ex != null) {
            if (ex instanceof Error) {
                throw (Error) ex;
            }
            if (ex instanceof RuntimeException) {
                throw (RuntimeException) ex;
            }
            ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
        }
    }

    /**
     * The sneaky part of sneaky throw, relying on generics limitations to evade
     * compiler complaints about rethrowing unchecked exceptions
     */
    @SuppressWarnings("unchecked")
    static <T extends Throwable> void uncheckedThrow(Throwable t) throws T {
        if (t != null) {
            throw (T) t; // rely on vacuous cast
        }
    }

    /**
     * Throws exception, if any, associated with the given status.
     */
    private void reportException(int s) {
        if (s == CANCELLED) {
            throw new CancellationException();
        }
        if (s == EXCEPTIONAL) {
            rethrow(getThrowableException());
        }
    }

    // public methods
    /**
     * Arranges to asynchronously execute this task in the pool the current task
     * is running in, if applicable, or using the {@link
     * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}. While it is
     * not necessarily enforced, it is a usage error to fork a task more than
     * once unless it has completed and been reinitialized. Subsequent
     * modifications to the state of this task or any data it operates on are
     * not necessarily consistently observable by any thread other than the one
     * executing it unless preceded by a call to {@link #join} or related
     * methods, or a call to {@link #isDone} returning {@code
     * true}.
     *
     * @return {@code this}, to simplify usage
     */
    public final ForkJoinTask<V> fork() {
        Thread t;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
            ((ForkJoinWorkerThread) t).workQueue.push(this);
        } else {
            ForkJoinPool.common.externalPush(this);
        }
        return this;
    }

    /**
     * Returns the result of the computation when it {@link #isDone is
     * done}. This method differs from {@link #get()} in that abnormal
     * completion results in {@code RuntimeException} or {@code Error}, not
     * {@code ExecutionException}, and that interrupts of the calling thread do
     * <em>not</em> cause the method to abruptly return by throwing {@code
     * InterruptedException}.
     *
     * @return the computed result
     */
    public final V join() {
        int s;
        if ((s = doJoin() & DONE_MASK) != NORMAL) {
            reportException(s);
        }
        return getRawResult();
    }

    /**
     * Commences performing this task, awaits its completion if necessary, and
     * returns its result, or throws an (unchecked) {@code RuntimeException} or
     * {@code Error} if the underlying computation did so.
     *
     * @return the computed result
     */
    public final V invoke() {
        int s;
        if ((s = doInvoke() & DONE_MASK) != NORMAL) {
            reportException(s);
        }
        return getRawResult();
    }

    /**
     * Forks the given tasks, returning when {@code isDone} holds for each task
     * or an (unchecked) exception is encountered, in which case the exception
     * is rethrown. If more than one task encounters an exception, then this
     * method throws any one of these exceptions. If any task encounters an
     * exception, the other may be cancelled. However, the execution status of
     * individual tasks is not guaranteed upon exceptional return. The status of
     * each task may be obtained using {@link
     * #getException()} and related methods to check if they have been
     * cancelled, completed normally or exceptionally, or left unprocessed.
     *
     * @param t1 the first task
     * @param t2 the second task
     * @throws NullPointerException if any task is null
     */
    public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
        int s1, s2;
        t2.fork();
        if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL) {
            t1.reportException(s1);
        }
        if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL) {
            t2.reportException(s2);
        }
    }

    /**
     * Forks the given tasks, returning when {@code isDone} holds for each task
     * or an (unchecked) exception is encountered, in which case the exception
     * is rethrown. If more than one task encounters an exception, then this
     * method throws any one of these exceptions. If any task encounters an
     * exception, others may be cancelled. However, the execution status of
     * individual tasks is not guaranteed upon exceptional return. The status of
     * each task may be obtained using {@link #getException()} and related
     * methods to check if they have been cancelled, completed normally or
     * exceptionally, or left unprocessed.
     *
     * @param tasks the tasks
     * @throws NullPointerException if any task is null
     */
    public static void invokeAll(ForkJoinTask<?>... tasks) {
        Throwable ex = null;
        int last = tasks.length - 1;
        for (int i = last; i >= 0; --i) {
            ForkJoinTask<?> t = tasks[i];
            if (t == null) {
                if (ex == null) {
                    ex = new NullPointerException();
                }
            } else if (i != 0) {
                t.fork();
            } else if (t.doInvoke() < NORMAL && ex == null) {
                ex = t.getException();
            }
        }
        for (int i = 1; i <= last; ++i) {
            ForkJoinTask<?> t = tasks[i];
            if (t != null) {
                if (ex != null) {
                    t.cancel(false);
                } else if (t.doJoin() < NORMAL) {
                    ex = t.getException();
                }
            }
        }
        if (ex != null) {
            rethrow(ex);
        }
    }

    /**
     * Forks all tasks in the specified collection, returning when
     * {@code isDone} holds for each task or an (unchecked) exception is
     * encountered, in which case the exception is rethrown. If more than one
     * task encounters an exception, then this method throws any one of these
     * exceptions. If any task encounters an exception, others may be cancelled.
     * However, the execution status of individual tasks is not guaranteed upon
     * exceptional return. The status of each task may be obtained using {@link
     * #getException()} and related methods to check if they have been
     * cancelled, completed normally or exceptionally, or left unprocessed.
     *
     * @param tasks the collection of tasks
     * @return the tasks argument, to simplify usage
     * @throws NullPointerException if tasks or any element are null
     */
    public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
        if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
            invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
            return tasks;
        }
        @SuppressWarnings("unchecked")
        List<? extends ForkJoinTask<?>> ts =
                (List<? extends ForkJoinTask<?>>) tasks;
        Throwable ex = null;
        int last = ts.size() - 1;
        for (int i = last; i >= 0; --i) {
            ForkJoinTask<?> t = ts.get(i);
            if (t == null) {
                if (ex == null) {
                    ex = new NullPointerException();
                }
            } else if (i != 0) {
                t.fork();
            } else if (t.doInvoke() < NORMAL && ex == null) {
                ex = t.getException();
            }
        }
        for (int i = 1; i <= last; ++i) {
            ForkJoinTask<?> t = ts.get(i);
            if (t != null) {
                if (ex != null) {
                    t.cancel(false);
                } else if (t.doJoin() < NORMAL) {
                    ex = t.getException();
                }
            }
        }
        if (ex != null) {
            rethrow(ex);
        }
        return tasks;
    }

    /**
     * Attempts to cancel execution of this task. This attempt will fail if the
     * task has already completed or could not be cancelled for some other
     * reason. If successful, and this task has not started when {@code cancel}
     * is called, execution of this task is suppressed. After this method
     * returns successfully, unless there is an intervening call to {@link
     * #reinitialize}, subsequent calls to {@link #isCancelled},
     * {@link #isDone}, and {@code cancel} will return {@code true} and calls to
     * {@link #join} and related methods will result in
     * {@code CancellationException}.
     *
     * <p>This method may be overridden in subclasses, but if so, must still
     * ensure that these properties hold. In particular, the {@code cancel}
     * method itself must not throw exceptions.
     *
     * <p>This method is designed to be invoked by <em>other</em>
     * tasks. To terminate the current task, you can just return or throw an
     * unchecked exception from its computation method, or invoke
     * {@link #completeExceptionally}.
     *
     * @param mayInterruptIfRunning this value has no effect in the default
     * implementation because interrupts are not used to control cancellation.
     *
     * @return {@code true} if this task is now cancelled
     */
    public boolean cancel(boolean mayInterruptIfRunning) {
        return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
    }

    public final boolean isDone() {
        return status < 0;
    }

    public final boolean isCancelled() {
        return (status & DONE_MASK) == CANCELLED;
    }

    /**
     * Returns {@code true} if this task threw an exception or was cancelled.
     *
     * @return {@code true} if this task threw an exception or was cancelled
     */
    public final boolean isCompletedAbnormally() {
        return status < NORMAL;
    }

    /**
     * Returns {@code true} if this task completed without throwing an exception
     * and was not cancelled.
     *
     * @return {@code true} if this task completed without throwing an exception
     * and was not cancelled
     */
    public final boolean isCompletedNormally() {
        return (status & DONE_MASK) == NORMAL;
    }

    /**
     * Returns the exception thrown by the base computation, or a
     * {@code CancellationException} if cancelled, or {@code null} if none or if
     * the method has not yet completed.
     *
     * @return the exception, or {@code null} if none
     */
    public final Throwable getException() {
        int s = status & DONE_MASK;
        return ((s >= NORMAL) ? null
                : (s == CANCELLED) ? new CancellationException()
                : getThrowableException());
    }

    /**
     * Completes this task abnormally, and if not already aborted or cancelled,
     * causes it to throw the given exception upon {@code join} and related
     * operations. This method may be used to induce exceptions in asynchronous
     * tasks, or to force completion of tasks that would not otherwise complete.
     * Its use in other situations is discouraged. This method is overridable,
     * but overridden versions must invoke {@code super} implementation to
     * maintain guarantees.
     *
     * @param ex the exception to throw. If this exception is not a
     * {@code RuntimeException} or {@code Error}, the actual exception thrown
     * will be a {@code RuntimeException} with cause {@code ex}.
     */
    public void completeExceptionally(Throwable ex) {
        setExceptionalCompletion((ex instanceof RuntimeException)
                || (ex instanceof Error) ? ex
                : new RuntimeException(ex));
    }

    /**
     * Completes this task, and if not already aborted or cancelled, returning
     * the given value as the result of subsequent invocations of {@code join}
     * and related operations. This method may be used to provide results for
     * asynchronous tasks, or to provide alternative handling for tasks that
     * would not otherwise complete normally. Its use in other situations is
     * discouraged. This method is overridable, but overridden versions must
     * invoke {@code super} implementation to maintain guarantees.
     *
     * @param value the result value for this task
     */
    public void complete(V value) {
        try {
            setRawResult(value);
        } catch (Throwable rex) {
            setExceptionalCompletion(rex);
            return;
        }
        setCompletion(NORMAL);
    }

    /**
     * Completes this task normally without setting a value. The most recent
     * value established by {@link #setRawResult} (or {@code
     * null} by default) will be returned as the result of subsequent
     * invocations of {@code join} and related operations.
     *
     * @since 1.8
     */
    public final void quietlyComplete() {
        setCompletion(NORMAL);
    }

    /**
     * Waits if necessary for the computation to complete, and then retrieves
     * its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an exception
     * @throws InterruptedException if the current thread is not a member of a
     * ForkJoinPool and was interrupted while waiting
     */
    public final V get() throws InterruptedException, ExecutionException {
        int s = (Thread.currentThread() instanceof ForkJoinWorkerThread)
                ? doJoin() : externalInterruptibleAwaitDone();
        Throwable ex;
        if ((s &= DONE_MASK) == CANCELLED) {
            throw new CancellationException();
        }
        if (s == EXCEPTIONAL && (ex = getThrowableException()) != null) {
            throw new ExecutionException(ex);
        }
        return getRawResult();
    }

    /**
     * Waits if necessary for at most the given time for the computation to
     * complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an exception
     * @throws InterruptedException if the current thread is not a member of a
     * ForkJoinPool and was interrupted while waiting
     * @throws TimeoutException if the wait timed out
     */
    public final V get(long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        }
        // Messy in part because we measure in nanosecs, but wait in millisecs
        int s;
        long ms;
        long ns = unit.toNanos(timeout);
        if ((s = status) >= 0 && ns > 0L) {
            long deadline = System.nanoTime() + ns;
            ForkJoinPool p = null;
            ForkJoinPool.WorkQueue w = null;
            Thread t = Thread.currentThread();
            if (t instanceof ForkJoinWorkerThread) {
                ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
                p = wt.pool;
                w = wt.workQueue;
                p.helpJoinOnce(w, this); // no retries on failure
            } else {
                ForkJoinPool.externalHelpJoin(this);
            }
            boolean canBlock = false;
            boolean interrupted = false;
            try {
                while ((s = status) >= 0) {
                    if (w != null && w.qlock < 0) {
                        cancelIgnoringExceptions(this);
                    } else if (!canBlock) {
                        if (p == null || p.tryCompensate()) {
                            canBlock = true;
                        }
                    } else {
                        if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L
                                && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                            synchronized (this) {
                                if (status >= 0) {
                                    try {
                                        wait(ms);
                                    } catch (InterruptedException ie) {
                                        if (p == null) {
                                            interrupted = true;
                                        }
                                    }
                                } else {
                                    notifyAll();
                                }
                            }
                        }
                        if ((s = status) < 0 || interrupted
                                || (ns = deadline - System.nanoTime()) <= 0L) {
                            break;
                        }
                    }
                }
            } finally {
                if (p != null && canBlock) {
                    p.incrementActiveCount();
                }
            }
            if (interrupted) {
                throw new InterruptedException();
            }
        }
        if ((s &= DONE_MASK) != NORMAL) {
            Throwable ex;
            if (s == CANCELLED) {
                throw new CancellationException();
            }
            if (s != EXCEPTIONAL) {
                throw new TimeoutException();
            }
            if ((ex = getThrowableException()) != null) {
                throw new ExecutionException(ex);
            }
        }
        return getRawResult();
    }

    /**
     * Joins this task, without returning its result or throwing its exception.
     * This method may be useful when processing collections of tasks when some
     * have been cancelled or otherwise known to have aborted.
     */
    public final void quietlyJoin() {
        doJoin();
    }

    /**
     * Commences performing this task and awaits its completion if necessary,
     * without returning its result or throwing its exception.
     */
    public final void quietlyInvoke() {
        doInvoke();
    }

    /**
     * Possibly executes tasks until the pool hosting the current task
     * {@link ForkJoinPool#isQuiescent is quiescent}. This method may be of use
     * in designs in which many tasks are forked, but none are explicitly
     * joined, instead executing them until all are processed.
     */
    public static void helpQuiesce() {
        Thread t;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
            ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
            wt.pool.helpQuiescePool(wt.workQueue);
        } else {
            ForkJoinPool.quiesceCommonPool();
        }
    }

    /**
     * Resets the internal bookkeeping state of this task, allowing a subsequent
     * {@code fork}. This method allows repeated reuse of this task, but only if
     * reuse occurs when this task has either never been forked, or has been
     * forked, then completed and all outstanding joins of this task have also
     * completed. Effects under any other usage conditions are not guaranteed.
     * This method may be useful when executing pre-constructed trees of
     * subtasks in loops.
     *
     * <p>Upon completion of this method, {@code isDone()} reports
     * {@code false}, and {@code getException()} reports {@code
     * null}. However, the value returned by {@code getRawResult} is unaffected.
     * To clear this value, you can invoke {@code
     * setRawResult(null)}.
     */
    public void reinitialize() {
        if ((status & DONE_MASK) == EXCEPTIONAL) {
            clearExceptionalCompletion();
        } else {
            status = 0;
        }
    }

    /**
     * Returns the pool hosting the current task execution, or null if this task
     * is executing outside of any ForkJoinPool.
     *
     * @see #inForkJoinPool
     * @return the pool, or {@code null} if none
     */
    public static ForkJoinPool getPool() {
        Thread t = Thread.currentThread();
        return (t instanceof ForkJoinWorkerThread)
                ? ((ForkJoinWorkerThread) t).pool : null;
    }

    /**
     * Returns {@code true} if the current thread is a {@link
     * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
     *
     * @return {@code true} if the current thread is a {@link
     * ForkJoinWorkerThread} executing as a ForkJoinPool computation, or
     * {@code false} otherwise
     */
    public static boolean inForkJoinPool() {
        return Thread.currentThread() instanceof ForkJoinWorkerThread;
    }

    /**
     * Tries to unschedule this task for execution. This method will typically
     * (but is not guaranteed to) succeed if this task is the most recently
     * forked task by the current thread, and has not commenced executing in
     * another thread. This method may be useful when arranging alternative
     * local processing of tasks that could have been, but were not, stolen.
     *
     * @return {@code true} if unforked
     */
    public boolean tryUnfork() {
        Thread t;
        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                ? ((ForkJoinWorkerThread) t).workQueue.tryUnpush(this)
                : ForkJoinPool.tryExternalUnpush(this));
    }

    /**
     * Returns an estimate of the number of tasks that have been forked by the
     * current worker thread but not yet executed. This value may be useful for
     * heuristic decisions about whether to fork other tasks.
     *
     * @return the number of tasks
     */
    public static int getQueuedTaskCount() {
        Thread t;
        ForkJoinPool.WorkQueue q;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
            q = ((ForkJoinWorkerThread) t).workQueue;
        } else {
            q = ForkJoinPool.commonSubmitterQueue();
        }
        return (q == null) ? 0 : q.queueSize();
    }

    /**
     * Returns an estimate of how many more locally queued tasks are held by the
     * current worker thread than there are other worker threads that might
     * steal them, or zero if this thread is not operating in a ForkJoinPool.
     * This value may be useful for heuristic decisions about whether to fork
     * other tasks. In many usages of ForkJoinTasks, at steady state, each
     * worker should aim to maintain a small constant surplus (for example, 3)
     * of tasks, and to process computations locally if this threshold is
     * exceeded.
     *
     * @return the surplus number of tasks, which may be negative
     */
    public static int getSurplusQueuedTaskCount() {
        return ForkJoinPool.getSurplusQueuedTaskCount();
    }

    // Extension methods
    /**
     * Returns the result that would be returned by {@link #join}, even if this
     * task completed abnormally, or {@code null} if this task is not known to
     * have been completed. This method is designed to aid debugging, as well as
     * to support extensions. Its use in any other context is discouraged.
     *
     * @return the result, or {@code null} if not completed
     */
    public abstract V getRawResult();

    /**
     * Forces the given value to be returned as a result. This method is
     * designed to support extensions, and should not in general be called
     * otherwise.
     *
     * @param value the value
     */
    protected abstract void setRawResult(V value);

    /**
     * Immediately performs the base action of this task and returns true if,
     * upon return from this method, this task is guaranteed to have completed
     * normally. This method may return false otherwise, to indicate that this
     * task is not necessarily complete (or is not known to be complete), for
     * example in asynchronous actions that require explicit invocations of
     * completion methods. This method may also throw an (unchecked) exception
     * to indicate abnormal exit. This method is designed to support extensions,
     * and should not in general be called otherwise.
     *
     * @return {@code true} if this task is known to have completed normally
     */
    protected abstract boolean exec();

    /**
     * Returns, but does not unschedule or execute, a task queued by the current
     * thread but not yet executed, if one is immediately available. There is no
     * guarantee that this task will actually be polled or executed next.
     * Conversely, this method may return null even if a task exists but cannot
     * be accessed without contention with other threads. This method is
     * designed primarily to support extensions, and is unlikely to be useful
     * otherwise.
     *
     * @return the next task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> peekNextLocalTask() {
        Thread t;
        ForkJoinPool.WorkQueue q;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
            q = ((ForkJoinWorkerThread) t).workQueue;
        } else {
            q = ForkJoinPool.commonSubmitterQueue();
        }
        return (q == null) ? null : q.peek();
    }

    /**
     * Unschedules and returns, without executing, the next task queued by the
     * current thread but not yet executed, if the current thread is operating
     * in a ForkJoinPool. This method is designed primarily to support
     * extensions, and is unlikely to be useful otherwise.
     *
     * @return the next task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> pollNextLocalTask() {
        Thread t;
        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                ? ((ForkJoinWorkerThread) t).workQueue.nextLocalTask()
                : null;
    }

    /**
     * If the current thread is operating in a ForkJoinPool, unschedules and
     * returns, without executing, the next task queued by the current thread
     * but not yet executed, if one is available, or if not available, a task
     * that was forked by some other thread, if available. Availability may be
     * transient, so a {@code null} result does not necessarily imply quiescence
     * of the pool this task is operating in. This method is designed primarily
     * to support extensions, and is unlikely to be useful otherwise.
     *
     * @return a task, or {@code null} if none are available
     */
    protected static ForkJoinTask<?> pollTask() {
        Thread t;
        ForkJoinWorkerThread wt;
        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
                ? (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue)
                : null;
    }

    // tag operations
    /**
     * Returns the tag for this task.
     *
     * @return the tag for this task
     * @since 1.8
     */
    public final short getForkJoinTaskTag() {
        return (short) status;
    }

    /**
     * Atomically sets the tag value for this task.
     *
     * @param tag the tag value
     * @return the previous value of the tag
     * @since 1.8
     */
    public final short setForkJoinTaskTag(short tag) {
        for (int s;;) {
            if (U.compareAndSwapInt(this, STATUS, s = status,
                    (s & ~SMASK) | (tag & SMASK))) {
                return (short) s;
            }
        }
    }

    /**
     * Atomically conditionally sets the tag value for this task. Among other
     * applications, tags can be used as visit markers in tasks operating on
     * graphs, as in methods that check: {@code
     * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} before
     * processing, otherwise exiting because the node has already been visited.
     *
     * @param e the expected tag value
     * @param tag the new tag value
     * @return true if successful; i.e., the current value was equal to e and is
     * now tag.
     * @since 1.8
     */
    public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
        for (int s;;) {
            if ((short) (s = status) != e) {
                return false;
            }
            if (U.compareAndSwapInt(this, STATUS, s,
                    (s & ~SMASK) | (tag & SMASK))) {
                return true;
            }
        }
    }

    /**
     * Adaptor for Runnables. This implements RunnableFuture to be compliant
     * with AbstractExecutorService constraints when used in ForkJoinPool.
     */
    static final class AdaptedRunnable<T> extends ForkJoinTask<T>
            implements RunnableFuture<T> {

        final Runnable runnable;
        T result;

        AdaptedRunnable(Runnable runnable, T result) {
            if (runnable == null) {
                throw new NullPointerException();
            }
            this.runnable = runnable;
            this.result = result; // OK to set this even before completion
        }

        public final T getRawResult() {
            return result;
        }

        public final void setRawResult(T v) {
            result = v;
        }

        public final boolean exec() {
            runnable.run();
            return true;
        }

        public final void run() {
            invoke();
        }
        private static final long serialVersionUID = 5232453952276885070L;
    }

    /**
     * Adaptor for Runnables without results
     */
    static final class AdaptedRunnableAction extends ForkJoinTask<Void>
            implements RunnableFuture<Void> {

        final Runnable runnable;

        AdaptedRunnableAction(Runnable runnable) {
            if (runnable == null) {
                throw new NullPointerException();
            }
            this.runnable = runnable;
        }

        public final Void getRawResult() {
            return null;
        }

        public final void setRawResult(Void v) {
        }

        public final boolean exec() {
            runnable.run();
            return true;
        }

        public final void run() {
            invoke();
        }
        private static final long serialVersionUID = 5232453952276885070L;
    }

    /**
     * Adaptor for Callables
     */
    static final class AdaptedCallable<T> extends ForkJoinTask<T>
            implements RunnableFuture<T> {

        final Callable<? extends T> callable;
        T result;

        AdaptedCallable(Callable<? extends T> callable) {
            if (callable == null) {
                throw new NullPointerException();
            }
            this.callable = callable;
        }

        public final T getRawResult() {
            return result;
        }

        public final void setRawResult(T v) {
            result = v;
        }

        public final boolean exec() {
            try {
                result = callable.call();
                return true;
            } catch (Error err) {
                throw err;
            } catch (RuntimeException rex) {
                throw rex;
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }
        }

        public final void run() {
            invoke();
        }
        private static final long serialVersionUID = 2838392045355241008L;
    }

    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code run} method
     * of the given {@code Runnable} as its action, and returns a null result
     * upon {@link #join}.
     *
     * @param runnable the runnable action
     * @return the task
     */
    public static ForkJoinTask<?> adapt(Runnable runnable) {
        return new AdaptedRunnableAction(runnable);
    }

    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code run} method
     * of the given {@code Runnable} as its action, and returns the given result
     * upon {@link #join}.
     *
     * @param runnable the runnable action
     * @param result the result upon completion
     * @return the task
     */
    public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
        return new AdaptedRunnable<T>(runnable, result);
    }

    /**
     * Returns a new {@code ForkJoinTask} that performs the {@code call} method
     * of the given {@code Callable} as its action, and returns its result upon
     * {@link #join}, translating any checked exceptions encountered into
     * {@code RuntimeException}.
     *
     * @param callable the callable action
     * @return the task
     */
    public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
        return new AdaptedCallable<T>(callable);
    }
    // Serialization support
    private static final long serialVersionUID = -7721805057305804111L;

    /**
     * Saves this task to a stream (that is, serializes it).
     *
     * @serialData the current run status and the exception thrown during
     * execution, or {@code null} if none
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        s.defaultWriteObject();
        s.writeObject(getException());
    }

    /**
     * Reconstitutes this task from a stream (that is, deserializes it).
     */
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        Object ex = s.readObject();
        if (ex != null) {
            setExceptionalCompletion((Throwable) ex);
        }
    }
    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long STATUS;

    static {
        exceptionTableLock = new ReentrantLock();
        exceptionTableRefQueue = new ReferenceQueue<Object>();
        exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
        try {
            U = getUnsafe();
            Class<?> k = ForkJoinTask.class;
            STATUS = U.objectFieldOffset(k.getDeclaredField("status"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating into a
     * jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException tryReflectionInstead) {
        }
        try {
            return java.security.AccessController.doPrivileged(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                public sun.misc.Unsafe run() throws Exception {
                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x)) {
                            return k.cast(x);
                        }
                    }
                    throw new NoSuchFieldError("the Unsafe");
                }
            });
        } catch (java.security.PrivilegedActionException e) {
            throw new RuntimeException("Could not initialize intrinsics",
                    e.getCause());
        }
    }
}
